This invention relates to and has among its objects the provision of a novel method for predicting the future acceptability of coarsely ground beef after regrinding and aerobic storage under specified conditions of time and temperature by measurement of the concentration of lactic acid in the sample prior to regrinding and storage.
Hamburger use is extensive in the United States. In 1982, Federally inspected ground beef totaled 3.2 billion pounds. A method for predicting the acceptability of ground beef could result in considerable economic savings by minimizing losses due to spoilage of ground beef. For example, a means to predict spoilage is important to ensure purchase of high quality meat for retail sale, for institutional use including the school lunch program and the military, and the like because it would ensure purchase of high quality meat by permitting judgment of purchase on predicted acceptability. Another important use of such a prediction method is so that a retailer, institution, etc. can make judgments for handling procedures of ground beef currently in stock so that quality control standards are met and losses due to spoilage are minimized. Another important use would be for regulatory purposes.
Refrigerated beef stored under aerobic conditions has only limited shelf life due to growth of aerobic spoilage flora in the meat, usually dominated by Pseudomonas spp., which are proteolytic bacteria and cause the "sweet-rotten" odor associated with spoiled meat. One extensively used handling practice to retard spoilage of ground beef during commercial storage and distribution from a central source is to coarse cut the beef (about 9.5 mm to 19.0 mm diameter die cut) and package it, with or without added carbon dioxide, in oxygen-impermeable casings in about 4.5 kg to 9.0 kg quantities called "chubs" or "keeper" packs. The meat is then shipped and stored under refrigeration until needed. In the anaerobically stored chub packs, there is a predominance of lactic acid-producing bacteria which are non-proteolytic, thus spoilage is retarded. Prior to sale or use, the coarsely ground beef is removed from the oxygen-impermeable casing and is reground in air to give the hamburger texture customarily used for consumption (about 5 mm diameter die cut) and its characteristic red color. This procedure causes the myoglobin in the meat to change from blue color to the red color of oxyhemoglobin associated with fresh meat. Also, the bacterial environment of the beef changes from predominantly lactic acid-producing bacteria to predominantly proteolytic microflora, such as Pseudomonas spp., and thus shelf life of the meat under conditions of aerobic storage is limited. Pseudomonas spp. which cause the "sweet-rotten" odor of spoiled meat grow under aerobic conditions which do not promote the growth of lactic acid bacteria.
A number of procedures have been proposed to correlate microbial and biochemical assays with meat spoilage but these have met with limited success. Bacterial counts are genenerally thought to be an indicator of early spoilage, with "off" odors becoming apparent when bacterial numbers reach approximately 10.sup.7 cells per gram of meat. Unfortunately, bacterial counts are time consuming, taking from 5 to 10 days for accurate assessment of psychrotrophic bacteria. There is also some disagreement as to whether total bacterial counts correlate with organoleptic appraisal of the meat product and whether total or differential counts can be used to assess future acceptability. Proteolytic bacteria, such as some Pseudomonas spp., will cause spoilage at lower numbers than lactic acid-producing organisms which can produce "acid/sour" spoilage due to buildup of lactic acid. Several tests other than total bacterial counts have been tried to measure the microbial quality of processed meat. These tests, which include indicator dye methods, extract release volume, release of ammonia, pH, and titratable acidity, have proven to be of limited value and are not in use. Reductase tests, using certain dyes which act as hydrogen acceptors in measurement of dehydrogenase levels, have been tried for determining spoilage in beef. These tests are not feasible for use with ground or minced beef products due to release of cellular reductones during the grinding process.
The relationship (positive correlation) between lactic acid concentration and bacterial spoilage in coarsely ground beef stored under anaerobic conditions where lactic acid bacteria predominate and where the activity of proteolytic spoilage bacteria is minimal was reported by P. S. Nassos et al. in Applied and Environmental Microbiology 46(4): 894-900 (1983). Spoilage was determined by organoleptic acceptability testing. The increased lactic acid concentration which develops in coarsely ground beef in oxygen-impermeable casings was thought by researchers to be partially responsible for prohibiting growth of Pseudomonas spp. and other proteolytic microflora. The relationship between lactic acid and growth of proteolytic Gram-negative microflora, however, appears to be complex. Research by others gives evidence of lactic acid as a possible promotor, rather than inhibitor of growth, as some Pseudomonas spp. will utilize glucose preferentially, but once glucose is exhausted in the media, will utilize lactic acid and amino acids for growth. Because of the complex and conflicting information between lactic acid and proteolytic bacteria, none of the known research provided any suggestion as to how to predict spoilage in coarsely ground beef once it has been reground and stored under aerobic conditions where proteolytic bacteria (Pseudomonas spp.) dominate the microflora.